The present invention relates to antennas suitable for use at subscriber sites in a television distribution system for receiving microwave signals.
Subscription television service is typically provided either by hardwired cable systems or by "wireless cable" over-the-air systems. Wireless cable systems generally transmit within multiple bands of microwave frequencies, e.g., the 2.15 to 2.162 GHz Multipoint Distribution System (MDS) Band, the 2.4 to 2.4835 GHz Industrial Scientific Medical (ISM) Band and the 2.5 to 2.686 GHz Multichannel Multipoint Distribution System/Instructional Television Fixed Service (MMDS/ITFS) Band. All such microwave links are subject to the detrimental effects of multipath and competing source interference. In order to reduce the effects of interfering sources while obtaining a low cost to performance ratio, it is desirable to control antenna parameters such as the side lobe level, the front-to-back ratio, and the cross polarization level.
A typical prior art dipole feed antenna 10 used in a television distribution system (as shown in FIG. 1A) consists of a parabolic wire grid reflector 12 fed by a dipole 14 with a metal splash plate 16 placed approximately a quarter wavelength (relative to the center frequency of its desired frequency band) away from the dipole 14 on the opposite side from the reflector 12. The dipole 14 is typically placed within a radome 18 and positioned in front of the reflector 12 with a hollow metal tube 20. The tube 20 typically accommodates either a coaxial cable 22 or a downconverter. U.S. Design Pat. No. 269,009 and 268,343 respectively show typical examples of reflectors and radomes found in the prior art.
This use of the dipole 14 with the splash plate 16 typically presents some difficulties for feeding the reflector 12. While the purpose of the splash plate 16 is to increase the sensitivity of the dipole 14 towards as compared to away from the reflector 12, i.e., the front-to-back ratio, the measured radiation pattern shows that typical front-to-back ratio of this type of dipole represents an undesirable signal loss due to the lack of sensitivity of the dipole 14 in the direction of the reflector 12. Another typical drawback to using the splash plate 16 near the dipole 14 is that it blocks a portion of the signal coming into the reflector 12, reducing its effective area as well as forming a discontinuity in the electric field distribution impinging on the reflector surface.
Another class of prior art antennas includes log periodic (LP) antennas as described in Chapter 14 of the "ANTENNA ENGINEERING HANDBOOK Third Edition" by Richard C. Johnson, which is herein incorporated by reference. FIG. 1B, a reproduction of FIGS. 14-32 of the aforementioned reference, shows a schematic diagram of a typical LP antenna 24 comprised of first and second electrically conductive feed lines 26 and 28 driven by a signal source 30 and a plurality of dipoles 32, 34, 36, 38, 40, 42, 44, 46 and 48 coupled to the feed lines 26 and 28. Each dipole, e.g., dipole 48, is formed from opposing dipole halves, e.g., dipole halves 50 and 52, that are respectively coupled at right angles to the feed lines 26 and 28. A significant feature of the LP antenna 24 is that a single line 54 connecting the end points of each of the dipoles defines a taper .alpha. which prescribes the performance of the LP antenna 24. LP antennas are typically used to achieve broad bandwidths, e.g., on the order of several decades. However, the radiation characteristics of LP antennas are not well suited for use as feeds for reflectors since they typically have low gain, a low front-to-back ratio and unequal beamwidths in the two principal planes. While the beamwidths can be made nearly equal by spreading the two halves of the LP antenna apart (see FIGS. 14-30 of the aforementioned reference), this approach typically increases blockage and cross polarization. Additionally, the phase center location of LP antennas typically moves with frequency along the LP antenna, e.g., between a center point 56 of dipole 32 and a center point 58 of dipole 48.